This invention concerns an inductive flowmeter for electrically conductive liquids, with a pair of magnet poles which are disposed diametrically outside a pipe carrying the liquid, and with electrodes which are conductively connected to the liquid.
Such inductive flow meters consist essentially of a magnet whose magnetic field penetrates the pipe carrying the liquid to be measured, and of two electrodes which are conductively connected to the liquid to be measured and at which the measured voltage which appears perpendicularly to the magnetic field, is taken off.
Customarily, the pipes consist of an electrically non-conductive material, e.g., a ceramic, and the electrodes go through this material and protrude into the liquid. Such pipes, however, are not suited for transporting liquid metals, e.g., liquid sodium in nuclear power plants; only metallic pipes, i.e., practically only steel pipes, can be considered here. Although there are also induced in these pipes currents which have an adverse effect on the flow measurement, these currents are negligibly small if, as is customary in nuclear power plants, the pipes have a relatively large diameter (of, say, 600 mm). The cross section of the wall of the pipe is then only small as compared to the inside cross section of the pipe, and the higher conductivity of the liquid metal as compared to the conductivity of the pipe material also has an effect in the direction toward a relative reduction of such short-circuit currents. In such a case, it is advisable, if only to avoid a weakening of the pipe and also sealing problems, to attach, e.g., weld, the electrodes to the outside of the pipe in an electrically conducting manner.
In such flow meters, the difficulty arises that, when the electrically conductive liquid enters the magnetic field, currents are induced in it which have such a direction that the magnetic field produced by them counteracts and partly cancels the magnetic field of the magnet. When leaving the magnetic field, the induced currents have such a direction that the magnetic field formed by them is superimposed on the original field and reinforces the same. This difficulty can be circumvented if the length of the magnetic field is made large in the direction of the pipe axis and the electrodes are mounted approximately in the center of the magnetic field (German Auslegeschrift No. 1,220,160). With large pipe diameters, such a solution can be realized only with difficulty, particularly if a permanent magnet is to be used as the magnet. Inductive flow meters with permanent magnets are used by preference in nuclear power plants, because of their reliability and freedom of maintenance. They may be an important part of the operational instrumentation, for instance, at radiation-exposed and poorly accessible points of the sodium loops, and may in some circumstances intervene in the safety system of the reactor. The reliability of the permanent magnets is based primarily on the fact that the magnetic field is constant, independently of time. If suitable magnet alloys are used, also the influence of the temperature involved, remains within the permissible measuring accuracy. Permanent-magnet flow meters have no parts subject to wear.
In the German Offenlegungsschrift No. 2,225,356, an inductive flow meter with a permanent magnet is therefore described, in which it was attempted to circumvent the above-described difficulties of the weakening and reinforcing of the magnetic field at the ends of the magnet's field, by using a multiplicity of electrodes which are distributed on the pipe in the direction of the pipe's longitudinal axis. Their output voltages or currents are added together weighted. In such an arrangement a multiplicity of electrodes must be connected in parallel in order to obtain sufficient linearity between the measured signal and the flow rate, with short magnets and large pipe diameters. For this purpose, a very long straight section of pipe is required, which is not available in many cases. It is also difficult to fulfill the redundancy of the measuring signal, which is required for the safety system of a reactor.
In inductive flow-rate measurement, a further difficulty stems from the fact that the conductivity of the liquid changes with temperature. For most non-metallic liquids, it increases with temperature, and for metallic liquids, e.g., liquid sodium, it decreases. However, with increasing electric conductivity, the intensity of the interfering supplemental magnetic fields which appear at the ends of the magnet, increases, so that a separate characteristic is obtained for every temperature of the liquid. Particularly with large pipe diameters and a short magnet length, this phenomenon is very pronounced.
Care must further be taken in the design of inductive flowmeters that the measured signal is as large as possible and free of interference signals. In the arrangement according to U.S. Pat. No. 2,722,122, there is therefore provided, in addition to the pair of electrodes for the measuring signal, a second pair of electrodes which is mounted at those points of the pipe wall which are adjacent to the magnet poles, so that the line connecting the poles runs in the direction of the magnetic field lines. No measuring signal appears at these electrodes. The connecting lines of these auxiliary electrodes and the measuring electrodes are brought to an evaluation device in such a manner that stray interference signals are canceled.
It is an object of the present invention to create an inductive flowmeter which yields a large useful signal that depends linearly on the flow velocity. Its characteristic is to be independent of variations of the electric conductivity of the liquid to be measured and thus, of the temperature of the liquid.